The disclosure of Japanese Patent Application No. HEI 11-287256 filed on Oct. 7, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a method of controlling a power output apparatus including a prime mover, such as an engine, and an electric motor or motors, as power sources, and more particularly to a method of controlling such a power output apparatus that is provided with a coupling device capable of coupling a rotary shaft of the electric motor to at least one of a drive shaft and an output shaft of the prime mover.
2. Discussion of Related Art
In recent years, hybrid vehicles, such as that as disclosed in Japanese laid-open Patent Publication No. HEI 9-47094, have been proposed wherein a power output apparatus having an engine and an electric motor(s) as power sources is installed on the vehicle. In such a hybrid vehicle, the power output apparatus installed on the vehicle transmits a part of the power generated by the engine to a drive shaft through a power adjusting device, and converts the remaining power into electric power in a regenerative manner. The electric power may be stored in a battery, or used for driving the electric motor as a power source other than the engine. During the power transmitting process as described above, the power output apparatus controls the power adjusting device and the electric motor so as to output the power generated by the engine to the drive shaft at a desired rotation speed with desired torque. This makes it possible to operate the engine with a high operating efficiency by selecting an appropriate engine operating point, irrespective of the required output to be transmitted from the drive shaft, and therefore the hybrid vehicle is excellent in terms of energy saving or resource conservation and exhaust emission control, as compared with a conventional vehicle having only an engine as a power source.
In the power output apparatus as described above, the rotary shaft of the electric motor may be coupled to the drive shaft or the output shaft of the engine. In a first coupling state in which the rotary shaft of the electric motor is coupled to the drive shaft, so-called power circulation, or transmission of power from the downstream side to the upstream side, occurs during overdrive operations (overdrive running) in which the rotation speed of the drive shaft is higher than the engine speed, with a result of a reduced amount of power being generated by the engine and effectively transmitted to the drive shaft. Thus, the power output apparatus placed in the above coupling state exhibits a higher operating efficiency during underdrive operations (underdrive running) in which the rotation speed of the drive shaft is lower than the engine speed, as compared with the operating efficiency during overdrive operation. In a second coupling state in which the rotary shaft of the electric motor is coupled to the output shaft of the engine, on the other hand, the power effectively transmitted to the drive shaft is reduced due to the above-described power circulation occurring during underdrive operations (underdrive running). Thus, the power output apparatus exhibits a higher operating efficiency during overdrive operations (overdrive running), as compared with that during underdrive operations (underdrive running).
In view of the above situations, a power output apparatus has been proposed which is capable of switching the coupling state of the rotary shaft of the electric motor, namely, coupling the rotary shaft to a selected one of the drive shaft and the output shaft of the engine, and an example of such an apparatus is disclosed in Japanese laid-open Patent Publication No. HEI 10-271749. The power output apparatus of this type includes a first clutch that permits coupling and uncoupling of the rotary shaft of the electric motor to and from the output shaft of the engine, and a second clutch that permits coupling and uncoupling of the rotary shaft of the electric motor to and from the drive shaft. Where the rotation speed of the drive shaft becomes lower than the engine speed (during underdrive running), the first clutch is released while the second clutch is engaged or applied, so that the rotary shaft of the electric motor is coupled to the drive shaft. Where the rotation speed of the drive shaft becomes higher than the engine speed (during overdrive running), on the other hand, the first clutch is engaged while the second clutch is released, so that the rotary shaft of the electric motor is coupled to the output shaft of the engine. In this manner, the power output apparatus operates with a sufficiently high efficiency during both underdrive operations and overdrive operations.
In the following description, the coupling state in which the rotary shaft of the electric motor is coupled to the drive shaft will be called xe2x80x9cunderdrive (UD) couplingxe2x80x9d, and the coupling state in which the rotary shaft of the electric motor is coupled to the output shaft of the engine will be called xe2x80x9coverdrive (OD) couplingxe2x80x9d.
In the hybrid vehicle on which the power output apparatus as described above is installed, the coupling state of the rotary shaft of the electric motor is changed in the following manner depending upon running conditions of the vehicle.
When the vehicle is started from its rest state, the vehicle is always started while UD coupling is established, so that the drive shaft can generate a large driving torque. With UTD coupling thus established, the sum of the torque from the engine and the torque from the electric motor is applied as driving torque to the drive shaft.
While the vehicle is running at a high-speed, on the other hand, OD coupling is established so as to reduce loss in the power output apparatus (i.e., loss in the engine, electric motor, power adjusting device and others), and improve the fuel economy during high-speed running.
When the vehicle starts from rest with UTD coupling established as described above, the vehicle keeps running only by means of the electric motor, without starting the engine. More specifically, the electric motor produces driving torque to be applied to the drive shaft, using electric power stored in a battery. With this arrangement, the vehicle is able to run without using a low-speed operating region in which the engine operates with a poor efficiency, thus assuring improved fuel economy.
Here, the mode in which the vehicle runs only by means of the electric motor without using the engine will be called xe2x80x9cEV modexe2x80x9d (or xe2x80x9cEV runningxe2x80x9d), and the mode in which the engine is started and the vehicle runs using both the engine and the electric motor will be called xe2x80x9cHV modexe2x80x9d (or xe2x80x9cHV runningxe2x80x9d).
FIG. 26 is a diagram showing operating patterns in which a hybrid vehicle starts from its rest state. In FIG. 26, the vertical axis indicates torque, and the horizontal axis indicates rotation speed. FIG. 26 shows operating characteristics of a power output apparatus.
In FIG. 26, curve LIM is the maximum output line of the power output apparatus. Accordingly, a region defined by the vertical axis indicating the torque, the horizontal axis indicating the rotation speed, and the curve LIM indicates a range in which the operating point of the drive shaft can be located, namely, the operating region of the power output apparatus. The operating point is expressed as a combination of the torque and the rotation speed.
The curve EL is an operating line used when determining the target operating point of the engine. The engine exhibits the highest efficiency on this operating line EL, and the engine operates with optimum fuel economy if the target operating point of the engine is determined according to this operating line EL.
In general, the operating line of the engine is a boundary on which the engine speed and the rotation speed of the drive shaft are equal to each other. Accordingly, the engine operates with the rotary shaft of the electric motor held in the OD coupling state in a region in which the torque is higher than the operating line EL, and operates with the rotary shaft held in the UD coupling state in a region in which the torque is lower than the operating line EL. The region in which the torque is lower than the operating line EL will be hereinafter called xe2x80x9coverdrive regionxe2x80x9d (xe2x80x9cOD regionxe2x80x9d), and the region in which the torque is higher than the operating line EL will be called xe2x80x9cunderdrive regionxe2x80x9d (xe2x80x9cUD regionxe2x80x9d). Also, the operating line EL may be called xe2x80x9cUD/OD region boundaryxe2x80x9d when appropriate.
In FIG. 26, curve ESU is an engine start judgement line based on which it is determined whether the engine should be started or not. In general, the engine is at rest or stopped when the operating point of the drive shaft lies in a region on the lower left-hand side of the engine start judgement line ESU, and the engine is started with fuel supplied to the engine when the operating point of the drive shaft passes the engine start judgement line ESU and enters a region on the upper right-hand side of the judgement line ESU. In this case, therefore, the engine is started under an engine start condition that the operating point of the drive shaft lies in the upper right-hand region with respect to the engine start judgement line ESU.
Each of curve DL1 and curve DL2 represents a path followed by the operating point of the drive shaft when the vehicle starts from its rest state and runs.
In FIG. 26, xe2x80x9cDDxe2x80x9d denotes a curve on which the running resistance is equal to 0%. Each of P1xcx9cP6 denote curves at which power is equal to a certain value. The curves P1xcx9cP6 are plotted so that the power increases in the order of P1, P2, . . . P5, P6.
The curve DL1 and the curve DL2 represent two different cases in which the vehicle starts from its rest state and runs in their respective manners. In either case, the vehicle starts from rest with the rotating shaft of the electric motor held in the UD coupling state, as described above. Also, when the vehicle starts from rest, the engine is not started, and the vehicle runs in the EV mode (EV running) only by means of the electric motor.
In the case of curve DL1, the driver of the vehicle depresses an accelerator pedal by a large degree after the start of the vehicle, with a result of an increased required power to be generated by the engine, whereby the operating point of the drive shaft passes the engine start judgement line ESU (namely, the engine start condition is satisfied). As a result, fuel is supplied to the engine so as to start the engine, and the vehicle runs in the HV mode (HV running) using both the engine and the electric motor. If the vehicle is further accelerated until the operating point of the drive shaft passes the engine operating line (i.e., UD/OD region boundary) EL, and enters from the UD region into the OD region, the coupling state of the rotary shaft of the electric motor switches from UD coupling to OD coupling. A method of switching the coupling state of the rotary shaft of the electric motor from UD coupling to OD coupling during HV running of the vehicle is disclosed in, for example, Japanese laid-open Patent Publication No. HEI 10-271749.
In the case of curve DL2, on the other hand, the vehicle is started, and then runs in the EV mode only by means of the electric motor while it is being accelerated at a low rate. In this case, the operating point of the drive shaft does not pass the engine start judgement line ESU, and therefore the vehicle is kept running in the EV mode with the engine being at rest. If the vehicle is then accelerated, the operating point of the drive shaft leaves the UD region, passes the operating line of the engine (i.e., the UD/OD region boundary) EL, and enters the OD region. If the vehicle is further accelerated, the operating point of the drive shaft passes the engine start judgement line ESU within the OD region (namely, satisfies the engine start condition).
As described above, the operation in the case where the operating point of the drive shaft passes the UD/OD region boundary EL and enters from the UD region into the OD region during HV running of the vehicle is disclosed in the above-identified publications. However, none of the above-identified publications discloses the operation in the case where the vehicle is accelerated so that the operating point of the drive shaft passes the UD/OD region boundary EL and enters the OD region from the UD region during EV running, and where the engine start condition is subsequently satisfied, as in the case of curve DL2. Thus, substantially no consideration had been conventionally given to the case as indicated by curve DL2.
It is therefore an object of the present invention to provide a method of controlling a power output apparatus that is able to switch the coupling state of a rotary shaft of an electric motor and start the engine in appropriate timing in the case where the operating point of the drive shaft enters from a UD region into an OD region during EV running, and where the engine start condition is subsequently started.
To accomplish at least a part of the above object, the present invention provides a first method of controlling a power output apparatus which includes an engine including an output shaft, a drive shaft that outputs power, a power adjusting device that includes a first electric motor and is coupled with the output shaft and the drive shaft, the power adjusting device being capable of adjusting at least the power transmitted to the drive shaft by means of the first electric motor, a second electric motor having a rotary shaft, and a coupling device operable to couple the rotary shaft of the second electric motor to at least one of the drive shaft and the output shaft, the power output apparatus having an operating region represented by the relationship between the torque and the speed of rotation, the operating region being divided by a predetermined boundary into a first region in which the rotary shaft of the second electric motor is coupled with the drive shaft, and a second region in which the rotary shaft of the second electric motor is coupled with the output shaft, the method comprising the steps of: (a) operating the second electric motor while keeping the engine stopped when an operating point of the drive shaft lies in the first region and the rotary shaft of the second electric motor is coupled with the drive shaft through the coupling device; and (b) when the operating point of the drive shaft passes the boundary and enters the second region, starting fuel supply to the engine so as to start the engine, while at the same time switching coupling of the rotary shaft of the second electric motor from a first coupling state in which the rotary shaft is coupled with the drive shaft, to a second coupling state in which the rotary shaft is coupled with the output shaft of the engine.
In the first control method as described above, when the operating point of the drive shaft passes the predetermined boundary and enters the second region while operating the second electric motor with the engine being stopped, fuel begins to be supplied to the engine so as to start the engine, and the coupling device is caused to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft of the engine.
According to the first control method, therefore, the drive shaft is initially rotated by operating only the second electric motor, and, when the operating point of the drive shaft enters from the first region into the second region, the rotary shaft of the second electric motor is immediately coupled to the output shaft of the engine, so that the drive shaft can be rotated by operating both the engine and the second electric motor.
Suppose that the above-indicated boundary is the operating line of the engine, and the first region is the UD region as described above, while the second region is the OD region, and that the power output apparatus as described above is used for running a motor vehicle. In this case, when the operating point of the drive shaft enters the OD region from the UD region while the vehicle is running in the EV mode using only the second electric motor, the rotary shaft of the second electric motor is brought into the OD coupling state in which the rotary shaft is coupled with the drive shaft, and both the engine and the second electric motor are operated so as to run the vehicle in the HV mode.
In one preferred form of the first control method of the present invention, the process (b) includes the steps of: starting fuel supply to the engine so as to start the engine when the operating point of the drive shaft passes the boundary and enters the second region; controlling the first electric motor and the engine so that the rotation speed and torque of the output shaft of the engine become substantially equal to those of the drive shaft after the engine is started; and causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft, after the rotation speed and torque of the output shaft become substantially equal to those of the drive shaft.
Since coupling of the rotary shaft of the second electric motor is switched to the second coupling state after the rotation speed and torque of the output shaft are made substantially equal to those of the drive shaft after the start of the engine, the switching operation can be accomplished without suffering from any shock.
In the present specification, the statement that xe2x80x9cthe rotation speed and torque of the output shaft are made substantially equal to those of the drive shaftxe2x80x9d should be interpreted to include the case where a difference in the rotation speed between the output shaft and the drive shaft falls within a certain allowable range, and the case where a difference in the torque between the output shaft and the drive shaft falls within a certain allowable range.
In another preferred form of the first control method of the present invention, the above-indicated step (b) includes the steps of: controlling the first electric motor and the engine so that the rotation speed and torque of the output shaft become substantially equal to those of the drive shaft when the operating point of the drive shaft passes the boundary and enters the second region; causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft, after the rotation speed and torque of the output shaft become substantially equal to those of the drive shaft; and starting fuel supply to the engine so as to start the engine after switching to the second coupling state in which the rotary shaft of the second electric motor is coupled with the output shaft.
Since coupling of the rotary shaft of the second electric motor is switched to the second coupling state after the rotation speed and torque of the output shaft of the engine are made substantially equal to those of the drive shaft, the switching operation can be accomplished without suffering from any shock. Also, the speed of rotation of the engine does not rapidly increase upon coupling of the rotary shaft of the second electric motor with the output shaft since the speed of rotation of the output shaft has already increased to about the rotation speed of the drive shaft. This makes it possible to suppress torque variations and vibration that would otherwise occur due to a rapid increase in the engine speed. In addition, a coupling device having a small coupling capacity or capability (i.e., such a coupling device that is able to couple two elements having a small difference in the maximum speed of rotation) may be used as the above-indicated coupling device for switching coupling of the rotary shaft of the second electric motor. Furthermore, coupling of the rotary shaft of the second electric motor is switched before the start of the engine, and therefore the switching operation can be smoothly accomplished without being affected by variations in the torque and rotation speed that would occur immediately after the start of the engine.
In the present specification, the statement that xe2x80x9cthe rotation speed of the output shaft is made substantially equal to that of the drive shaftxe2x80x9d should be interpreted to include the case where a difference in the rotation speed between the output shaft and the drive shaft falls within a certain allowable range.
The present invention also provides a second method of controlling the power output apparatus as described above, comprising the steps of: (a) operating the second electric motor while keeping the engine stopped so that the second electric motor outputs driving torque to the drive shaft when an operating point of the drive shaft lies in the first region and the rotary shaft of the second electric motor is coupled with the drive shaft through the coupling device; (b) causing the coupling device to switch coupling of the rotary shaft of the second electric motor from a first coupling state in which the rotary shaft is coupled with the drive shaft, to a second coupling state in which the rotary shaft is coupled with the output shaft of the engine, when the operating point of the drive shaft passes the boundary and enters the second region; and (c) after switching to the second coupling state in which the rotary shaft of the second electric motor is coupled with the output shaft of the engine, causing the first electric motor to output driving torque to the drive shaft, instead of the second electric motor, while causing the second electric motor to cancel reactive torque generated by the first electric motor at the output shaft.
In the second control method as described above, when the operating point of the driving shaft passes the above-indicated boundary and enters the second region while the second electric motor is operating to output driving torque to the drive shaft with the engine stopped, coupling of the rotary shaft of the second electric motor is switched from the first coupling state with the drive shaft to the second coupling state with the output shaft of the engine. After the switching, the first electric motor operates to output driving torque to the drive shaft, and the second electric motor operates to cancel reactive torque that is generated by the first electric motor at the output shaft.
According to the second control method, coupling of the rotary shaft of the second electric motor is switched from the first coupling state with the drive shaft to the second coupling state with the output shaft when the operating point of the drive shaft enters from the first region into the second region. In this case, driving torque can be still outputted by the electric motor with the engine stopped, though the first electric motor, rather than the second electric motor, outputs the driving torque to the drive shaft.
Suppose that the above-indicated boundary is the operating line of the engine, and the first region is the UD region as described above, while the second region is the OD region, and that the power output apparatus as described above is used for running a motor vehicle. In this case, even when the operating point of the drive shaft enters the OD region from the UD region, the vehicle that has been running in the EV mode using the second electric motor continues to run in the EV mode, using the first electric motor.
In one preferred form of the second control method of the present invention, the step (b) comprises the steps of: controlling the first electric motor so that the rotation speed and torque of the output shaft become substantially equal to those of the drive shaft when the operating point of the drive shaft passes the boundary and enters the second region; and causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft, after the rotation speed and torque of the output shaft become substantially equal to those of the drive shaft.
The present invention also provides a third method of controlling the power output apparatus as described above, comprising the steps of: (a) operating the second electric motor while keeping the engine stopped when an operating point of the drive shaft lies in the first region and the rotary shaft of the second electric motor is coupled with the drive shaft through the coupling device; (b) when the operating point of the drive shaft passes the boundary and enters the second region, keeping the rotary shaft of the second electric motor coupled with the drive shaft through the coupling device, and operating the second electric motor while keeping the engine stopped; and (c) when a target power to be outputted from the drive shaft satisfies a predetermined condition, starting fuel supply to the engine so as to start the engine, and causing the coupling device to switch coupling of the rotary shaft of the second electric motor from a first coupling state in which the rotary shaft is coupled with the drive shaft, to a second coupling state in which the rotary shaft is coupled with the output shaft of the engine.
In the third control method as described above, even when the operating point of the drive shaft passes the above-indicated boundary and enters the second region while the second electric motor is being operated with the engine stopped, the coupling device maintains coupling of the rotary shaft of the second electric motor with the drive shaft, and the second electric motor continues to be operated with the engine stopped. When the target power to be outputted from the drive shaft subsequently satisfies a predetermined condition, fuel begins to be supplied to the engine so as to start the engine, and the coupling device is caused to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft of the engine.
According to the third control method, switching of the rotary shaft of the second electric motor does not take place while the second electric motor is being operated with the engine stopped, even if the operating point of the drive shaft enters from the first region into the second region. Accordingly, the drive shaft can be smoothly rotated over a wide range of the speed of rotation, without causing torque variations and vibration that would otherwise occur upon switching. When the target power subsequently satisfies the predetermined condition, the rotary shaft of the second electric motor is immediately coupled to the output shaft of the engine so that the operating state in which the drive shaft is rotated only by means of the second electric motor is shifted to the operating state in which the drive shaft is rotated by operating both the engine and the second electric motor.
Suppose that the above-indicated boundary is the operating line of the engine, and the first region is the UD region as described above, while the second region is the OD region, and that the power output apparatus as described above is used for running a motor vehicle. In this case, while the vehicle is running in the EV mode with only the second electric motor operated, the coupling of the rotary shaft of the second electric motor is not switched to OD coupling even when the operating point of the drive shaft enters from the UD region into the OD region. Thus, the control method of the present invention can assure smooth EV running of the vehicle over a wide vehicle speed range, without causing any torque variation and vibration. If the target power subsequently satisfies the predetermined condition, the coupling state of the rotary shaft of the second electric motor is switched to OD coupling, and the EV running of the vehicle can be immediately switched to the HV running.
In one preferred form of the third control method of the present invention, the step (c) comprises the steps of: starting fuel supply to the engine so as to start the engine when the target power satisfies the predetermined condition; controlling the first electric motor and the engine so that the rotation speed and torque of the output shaft of the engine become substantially equal to those of the drive shaft after the engine is started; and causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state to the second coupling state, after the rotation speed and torque of the output shaft become substantially equal to those of the drive shaft.
Since coupling of the rotary shaft of the second electric motor is switched to the second coupling state after the rotation speed and torque of the output shaft are made substantially equal to those of the drive shaft after the start of the engine, the switching operation can be accomplished without suffering from any shock.
In another preferred form of the third control method of the present invention, the step (c) comprises the steps of: controlling the first electric motor so that the rotation speed of the output shaft of the engine becomes substantially equal to that of the drive shaft when the target power satisfies the predetermined condition; causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state to the second coupling state, after the rotation speed of the output shaft of the engine becomes substantially equal to that of the drive shaft; and starting fuel supply to the engine so as to start the engine after switching to the second coupling state in which the rotary shaft of the second electric motor is coupled with the rotary shaft.
Since coupling of the rotary shaft of the second electric motor is switched in the manner as described above, the switching operation can be accomplished without suffering from any shock. Also, the speed of rotation of the engine does not rapidly increase upon coupling of the rotary shaft of the second electric motor with the output shaft, and therefore torque variations and vibration can be suppressed or prevented. Furthermore, a coupling device having a small coupling capacity or capability can be used as the coupling device of the power output apparatus. Moreover, coupling of the rotary shaft of the second electric motor is switched before the start of the engine, and therefore the switching operation can be smoothly accomplished without being affected by variations in the torque and rotation speed that would occur immediately after the start of the engine.
Preferably, the third control method of the present invention further includes the step of: (d) causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft when the rotation speed of the rotary shaft of the second electric motor exceeds a predetermined particular speed.
In the third control method as described above, while the second electric motor is operating with the engine stopped, the rotary shaft of the second electric motor is kept coupled with the drive shaft with no switching of coupling states even when the operating point of the drive shaft enters from the first region into the second region. It is, however, to be noted that the speed of rotation of the second electric power is limited to be equal to or lower than a permissible maximum speed, which makes it difficult to increase the speed of rotation of the drive shaft to be higher than the permissible maximum speed since the rotation speed of the drive shaft is limited by the second electric motor. If the above-indicated predetermined particular speed is set to be equal to or lower than the permissible maximum speed, for example, coupling of the rotary shaft of the second electric motor is switched from the first coupling state with the drive shaft to the second coupling state with the output shaft when the rotation speed of the rotary shaft of the second electric motor exceeds the particular speed, so that the rotation speed of the drive shaft is no longer limited or restricted by the second electric motor. This makes it possible to increase the rotation speed of the drive shaft to be higher than the permissible maximum speed.
In one preferred form of the third control method of the present invention, the above-indicated step (d) comprises the steps of: controlling the first electric motor so that the rotation speed of the output shaft of the engine becomes substantially equal to that of the drive shaft when the rotation speed of the rotary shaft of the second electric motor exceeds the predetermined speed; and causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft after the rotation speed of the output shaft becomes substantially equal to that of the drive shaft.
Since coupling of the rotary shaft of the second electric motor is switched in the manner as described above, the switching operation can be accomplished without suffering from any shock. Also, the speed of rotation of the engine does not rapidly increase upon coupling of the rotary shaft of the second electric motor with the output shaft, and therefore torque variations and vibration can be suppressed or prevented. Furthermore, a coupling device having a small coupling capacity or capability can be used as the coupling device of the power output apparatus.
The present invention also provides a fourth method of controlling the power output apparatus as described above, comprising the steps of: (a) operating the second electric motor while keeping the engine stopped when an operating point of the drive shaft lies in the first region and the rotary shaft of the second electric motor is coupled with the drive shaft through the coupling device; (b) keeping the rotary shaft of the second electric motor coupled with the drive shaft through the coupling device, and operating the second electric motor while keeping the engine stopped, when the operating point of the drive shaft passes the boundary and enters the second region; (c) controlling the first electric motor so that the rotation speed of the output shaft of the engine becomes substantially equal to that of the drive shaft when the operating point of the drive shaft passes the boundary and enters the second region; and (d) when a target power to be outputted from the drive shaft satisfies a predetermined condition after the rotation speed of the output shaft of the engine is made substantially equal to that of the drive shaft, starting fuel supply to the engine so as to start the engine, and causing the coupling device to switch coupling of the rotary shaft of the second electric motor from a first coupling state in which the rotary shaft is coupled with the drive shaft, to a second coupling state in which the rotary shaft is coupled with the output shaft of the engine.
In the fourth control method as described above, when the operating point of the drive shaft passes the above-indicated boundary and enters the second region while the second electric motor is being operated with the engine stopped, the coupling device maintains coupling of the rotary shaft of the second electric motor with the drive shaft, and the second electric motor continues to be operated with the engine stopped. In the meantime, the first electric motor is controlled so that the rotation speed of the output shaft becomes substantially equal to that of the drive shaft. When the target power to be outputted from the drive shaft satisfies a predetermined condition after the rotation speed of the output shaft is made substantially equal to that of the drive shaft, fuel supply to the engine is started thereby to start the engine, and the coupling device is caused to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft of the engine.
According to the fourth control method, switching of the rotary shaft of the second electric motor does not take place while the second electric motor is being operated with the engine stopped, even if the operating point of the drive shaft enters from the first region into the second region. Accordingly, the drive shaft can be smoothly rotated over a wide range of its rotation speed, without causing torque variations and vibration that would otherwise occur upon switching. Also, the rotation speed of the output shaft is controlled to be substantially equal to that of the drive shaft when the operating point of the drive shaft enters the second region. When the target power subsequently satisfies the predetermined condition, therefore, it takes only a moment to start the engine and switch coupling of the rotary shaft of the second electric motor, so that both the engine and the second electric motor can immediately operate to rotate the drive shaft.
Suppose that the above-indicated boundary is the operating line of the engine, and the first region is the UD region as described above, while the second region is the OD region, and that the power output apparatus as described above is used for running a motor vehicle. In this case, while the vehicle is running in the EV mode with only the second electric motor operated, the coupling of the rotary shaft of the second electric motor is not switched to OD coupling even when the operating point of the drive shaft enters from the UD region into the OD region. Thus, the control method of the present invention can assure smooth EV running of the vehicle over a wide vehicle speed range, without causing any torque variation and vibration. Also, the rotation speed of the output shaft of the engine is made substantially equal to that of the drive shaft when the operating point of the drive shaft enters the OD region. When the target power satisfies the predetermined condition, therefore, the engine can be instantly started, and coupling of the rotary shaft of the second electric motor can be instantly switched, so that the vehicle can immediately start running in the HV mode, thus assuring improved response of the driving torque of the vehicle to an acceleration demand operation of the driver.
In one preferred form of the fourth control method of the present invention, the step (d) comprises the steps of: starting fuel supply to the engine so as to start the engine when the target power satisfies the predetermined condition; and causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft after the engine is started.
Since the rotation speed of the output shaft is controlled to be substantially equal to that of the drive shaft after the operating point of the drive shaft enters the second region, the rotation speed of the output of the engine has already been made substantially equal to that of the drive shaft when the target power satisfies the predetermined condition. Accordingly, coupling of the rotary shaft of the second electric motor can be switched immediately after the start of the engine, without causing any shock upon switching.
In another preferred form of the fourth control method of the present invention, the step (d) comprises the steps of: causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft when the target power satisfies the predetermined condition; and starting fuel supply to the engine so as to start the engine after switching to the second coupling state in which the rotary shaft of the second electric motor is coupled with the output shaft.
Since the rotation speed of the output shaft is controlled to be substantially equal to that of the drive shaft after the operating point of the drive shaft enters the second region, coupling of the rotary shaft of the second electric motor can be switched any time without suffering from any shock. Also, the speed of rotation of the engine does not rapidly increase, and torque variations and vibration can be suppressed. Furthermore, a coupling device having a small coupling capacity or capability may be used as the coupling device of the power output apparatus. Moreover, coupling of the rotary shaft of the second electric motor is switched before the start of the engine, and therefore the switching operation can be smoothly accomplished without being affected by variations in the torque and rotation speed that would occur immediately after the start of the engine.
Preferably, the fourth control method of the present invention further comprises the step of: (e) causing the coupling device to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft when the rotation speed of the rotary shaft of the second electric motor exceeds a predetermined particular speed.
In the fourth control method as well as the third control method, while the second electric motor is operating with the engine stopped, the rotary shaft of the second electric motor is kept coupled with the drive shaft with no switching of coupling states even when the operating point of the drive shaft enters from the first region into the second region. In this case, the speed of rotation of the drive shaft is limited or restricted by the second electric motor, and it is thus impossible to increase the rotation speed of the drive shaft to be higher than the permissible maximum speed of the second electric motor. In view of this situation, coupling of the rotary shaft of the second electric motor is switched from the first coupling state with the drive shaft to the second coupling state with the output shaft when the rotation speed of the rotary shaft of the second electric motor exceeds the particular speed, so that the rotation speed of the drive shaft can be increased to be higher than the permissible maximum speed of the second motor.
The present invention also provides a fifth method of controlling the power output apparatus as described above, comprising the steps of: (a) operating the second electric motor while keeping the engine stopped when an operating point of the drive shaft lies in the first region and the rotary shaft of the second electric motor is coupled with the drive shaft through the coupling device; and (b) causing the coupling device to switch from a first coupling state in which the rotary shaft of the second electric motor is coupled with the drive shaft, to a second coupling state in which the rotary shaft is coupled with the output shaft of the engine, when the rotation speed of the rotary shaft of the second electric motor exceeds a predetermined particular speed.
In the fifth control method as described above, if the rotation speed of the rotary shaft of the second electric motor exceeds the particular speed where the rotary shaft of the second electric motor is coupled with the drive shaft, the coupling device is caused to switch coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft of the engine.
Since the rotation speed of the second electric motor is limited to be equal to or lower than the permissible maximum speed, the rotation speed of the drive shaft, which is restricted by the second electric motor, cannot be increased to be higher than the permissible maximum speed of the second motor if the rotary shaft of the second electric motor is kept coupled with the drive shaft. For example, the above-indicated particular speed is set to be equal to or lower than the permissible maximum speed. In this case, when the rotation speed of the rotary shaft of the second electric motor exceeds the particular speed, coupling of the rotary shaft of the second electric motor is switched from the first coupling state with the drive shaft to the second coupling state with the output shaft, so that the rotation speed of the drive shaft is no longer restricted or limited by the second electric motor, and thus is allowed to be increased to be higher than the permissible maximum speed.
In any of the first through fifth control methods of the present invention, when the coupling device switches coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft of the engine, the coupling device may couple the rotary shaft of the second electric motor to the output shaft of the engine while maintaining coupling of the rotary shaft of the second electric motor with the drive shaft, and subsequently uncouple the rotary shaft of the second electric motor from the drive shaft.
In the manner as described above, the rotary shaft of the second electric motor is coupled to the output shaft of the engine while being kept coupled with the drive shaft, so that the drive shaft and the output shaft can be mechanically directly coupled to each other. With this arrangement, the second electric motor is able to keep outputting driving torque to the drive shaft even during switching of coupling until the rotary shaft of the second electric motor is uncoupled or disengaged from the drive shaft.
In any of the first through third control methods of the present invention, in which coupling of the rotary shaft of the second electric motor is switched after the engine is started, the coupling device may initially uncouple the rotary shaft of the second electric motor from the drive shaft, and subsequently couple the rotary shaft of the second electric motor to the output shaft when the coupling device switches coupling of the rotary shaft of the second electric motor from the first coupling state with the drive shaft to the second coupling state with the output shaft of the engine.
Where the engine has already been started, the engine may operate to output driving torque to the drive shaft through the power output apparatus even if the rotary shaft of the second electric motor is uncoupled from the drive shaft. Thus, the rotary shaft of the second electric motor may be coupled to the output shaft after being uncoupled or disengaged from the drive shaft.
In any of the first through fifth control methods of the present invention, the power adjusting device may include, as the first electric motor, a doubled-rotor motor including a first rotor coupled to the output shaft and a second rotor coupled to the drive shaft, or may include a planetary gear train in addition to the first electric motor, which planetary gear train includes three rotary shafts that are respectively coupled to the output shaft, the drive shaft, and the rotary shaft of the first electric motor.
Thus, the power adjusting device may employ an electrically distributed type structure, using the doubled-rotor electric motor, or a mechanically distributed type structure, using planetary gears and others.
The present invention as explained above is applied to a method of controlling a power output apparatus. It is, however, possible to construct the present invention in the form of the power output apparatus itself that employs the control method, or various apparatuses, such as hybrid vehicles, on which such a power output apparatus is installed.